In an operating system that uses a Unix/Linux kernel, a large amount of system services run at the user space as a Daemon. Therefore, a better service will be provided by a process upgrade instead of a machine restart.
One process may communicate with another process or a kernel in various manners. For example, processes in the same machine may communicate with each other via an Inter Process Communication (IPC) technique. Processes in different machines may communicate with each other via Transmission Control Protocol (TCP)/User Datagram Protocol (UDP). In addition, a process may communicate with a kernel via a netlink socket. In a Unix/Linux kernel, these communication entities may be taken as File Descriptors (FD).